Embodiments of the present disclosure generally relate to a heat pump system and method, and, more particularly, to a system and method of efficiently operating a heat pump in a defrost mode.
An air-to-air heat pump, also known as an air source heat pump (“ASHP”), is generally a heating and cooling system that uses return air, regeneration air and/or outside air as a heat source and heat sink. An ASHP absorbs heat from the return/regeneration air and/or outside air and releases the heat to an enclosed space during a winter mode. Any heat source, including outside air, return air, or regeneration air) that has a temperature above absolute zero contains some heat. An ASHP pumps some of the heat from the heat source, for example.
Existing ASHPs generally include a supply air channel and a regeneration air channel. An energy exchange module typically extends between the supply air channel and the regeneration air channel. The energy exchange module transfers sensible and/or latent heat between the outside air in the supply air channel and the regeneration air in the regeneration air channel. The supply air channel and the regeneration air channel also typically include heat exchangers. The supply air channel includes a supply air heat exchanger and the regeneration air channel includes a regeneration air heat exchanger. The supply air heat exchanger and the regeneration air heat exchanger are in fluid communication through a refrigeration system that is configured to further transfer heat between the outside air and the regeneration air. In a winter mode, heat and/or moisture in the regeneration air is transferred to the outside air to generate heated and/or humidified supply air that is discharged from the supply air channel. During a summer mode, heat and moisture in the outside air is transferred to the regeneration air to generate cooled and dehumidified supply air that is discharged from the supply air channel.
However, conventional heat pump systems are not without their disadvantages. During winter modes, when the outside temperature drops below approximately 33-35° F., frost may form on the coils of the regeneration air heat exchanger. Accordingly, the heat pump system is typically shut down so that the coils can be defrosted. During the time period that the heat pump system is shut down, the building having the heat pump system typically is without a heat source or utilizes an auxiliary heat source. Additionally, during summer modes, the regeneration air heat exchanger may be required to supply large amounts of heat to the regeneration air. As a result, an efficiency of the regeneration air heat exchanger may substantially decrease.
Some systems include a pre-conditioning coil that is used to preheat the regeneration air during winter modes. The pre-conditioning coil is operable to prevent or minimize frost from forming on the regeneration air heat exchanger. However, the available energy in a refrigeration system is limited. The more energy that is provided to the pre-conditioning coil to prevent or minimize frost formation on the regeneration air heat exchanger, the less energy that is available for the supply air heat exchanger. Yet, if less energy is provided to the pre-conditioning coil, frost may accumulate more quickly on the regeneration heat exchanger. As frost accumulates on the regeneration heat exchanger, the efficiency of the heat exchanger decreases. As such, energy that is provided to the pre-conditioning coil that is configured to prevent or minimize frost formation on the regeneration air heat exchanger is energy that is not available for the supply air heat exchanger.
When ambient temperature is below 15° F., for example, much of the refrigerant is diverted to the pre-conditioning coil in order to prevent frost formation on the regeneration air heat exchanger. In order to capture useful heat in the regeneration air stream in an ASHP, the relative humidity ratio of the air leaving the regeneration air heat exchanger is typically above 80%. Indeed, the relative humidity ratio generally approaches 100% in order to be effective. In general, when a relative humidity ratio is above 80%, frost formation may increase. In order to provide effective frost prevention by way of a pre-conditioning coil, a relative humidity ratio of air leaving the pre-conditioning coil is typically limited to below 80% relative humidity. However, providing energy to the pre-conditioning coil in such a manner may decrease the overall system efficiency as useful or captured energy in the regeneration heat exchanger is simply expelled to the pre-conditioner and not the supply air heat exchanger. Alternatively, diverting only a portion of the hot gases to the pre-conditioner coil could effectively minimize and reduce frost formation on the regeneration heat exchanger and further increase the delay between required defrost cycles.
The primary defrosting method utilized in a typical ASHP is a reverse cycle defrost. In a typical reverse cycle defrost system, a reversing valve is switched from the heating position to the cooling position. The change in reversing valve position changes the flow of refrigerant in the refrigeration systems and sends all of the hot refrigerant to the regeneration/return air heat exchanger to defrost the ice that has accumulated. During the reverse cycle defrost, air flow across the regeneration air heat exchanger is interrupted to speed up the ice melting process. However, during this time, the supply air heat exchanger cools the supply air stream and discharges cold air to the enclosed space in winter. As such, an auxiliary heating source may be utilized to offset the supply air temperature drop.
Recent developments include attempts to fractionalize either the refrigeration cycle into smaller individual segments and/or sub-divide the air heat exchangers in the refrigeration circuit into multiple sub-sections. The primary goal is to alternate sub sections of the refrigeration system and/or heat exchangers with the objective being to continuously heat the supply air while sequentially defrosting portions of the regeneration/return air heat exchanger(s). In general, fractional systems may efficiently operate when the supply air is continuously heated with a compressorized system, while sequentially defrosting sub sections of the regeneration/return heat exchanger. However, when the regeneration heat exchanger sub-section is defrosted and a portion of the hot gas is diverted to the regeneration heat exchanger for melting ice build-up, there may be a reduction in the supply air temperature downstream from the supply air heat exchanger.
Further, when ice melts off the regeneration/return air heat exchanger and or sub-sections, the resulting water travels and runs to the bottom of the coil on the downstream side. The air on the downstream side of the regeneration air heat exchanger may be below freezing, so the draining water may freeze and subsequently accumulate in the drain pan. Heat wires in the drain pan may be used to prevent the re-freezing, yet using additional heating elements may increase the overall power consumption of the system.
Additionally, some heat exchanger fins may be corrugated and have a perforated surface configured to increase turbulence flow on the surface of the fin and ultimately increase heat transfer characteristics. While the corrugated fin surface increases heat exchange performance between the refrigerant and the air, the additional edges, surfaces, and smaller cavities increase the attractive forces (Van der Waals forces) of the water molecules to the fin surfaces. As such, during a defrost cycle, a significant quantity of water may remain within the coil fins and may not drain to and out the bottom of the coil. During the subsequent heating cycle, the remaining water may freeze in the regeneration/return air heat exchanger and reduce the amount of effective run time before the next defrost cycle.